This invention relates to an integral multilayer analytical element for the determination of ammonia or an ammonia-producing substance in a liquid sample. More particularly, this invention relates to an integral multilayer analytical element suitable for the determination of ammonia or an ammonia-producing substance such as creatinine or urea contained in a biological body fluid such as blood, urine or lymph.
The quantitative analysis of ammonia, creatinine, urea or the like is very important for the diagnosis of various diseases such as nephropathy, the inspection for the medical treatment course of the disease, and the inspection of renal functions.
A representative analytical method for an ammonia-producing substance comprises a process for producing ammonia from the ammonia-producing substance and a process for determining the produced ammonia. The analytical method of utilizing the conversion to ammonia has been widely utilized in the so-called wet analysis or the solution method. Recently, in the dry analysis using a dry analytical device represented by integral multilayer analytical elements, the analytical method of utilizing the conversion to ammonia has been applied or proposed.
As the process for producing ammonia from an ammonia-producing substance, a common method is of producing ammonia by the action of an enzyme. For example, creatinine in a biological body fluid is determined by utilizing creatinine deiminase (EC 3.5.4.21) which hydrolzes specifically creatinine in the body fluid to ammonia and N-methylhydantion. Urea nitrogen (BUN) in a biological body fluid is determined by utilizing urease which hydrolyzes urea to ammonia and carbon dioxide. In the above methods, since the ammonia-producing substance, which is an analyte, is a substrate of an enzyme, the substance is called also ammonia-producing substrate. Analytical methods of the ammonia-producing substance are described in various references, such as "Analytical Chemistry", 46, 246 (1974), "Climica Clinica Acta", 18, 409 (1967), "Rinsho Kagaku Bunseki III Gan-Chisso Seibun (Clinical Chemical Analysis III Nitrogen-Containing Components) 2nd Edition", Tokyo Kagaku Dojin, Tokyo, 13-14, 67-87 (1979) and "Rinsho Kensa (Journal of Medical Technology)" 5(6), 387-391 (1961).
Integral analytical elements usable for the analysis of ammonia or an ammonia-producing substrate are the integral multilayer analytical element described in Japanese Patent KOKOKU No. 58(1983)-19062 (U.S. Pat. No. Reissue No. 30,267), the integral multilayer analytical elements for the analysis of ammonia or an ammonia-producing substrate disclosed in U.S. Pat. No. 4,548,906 (Japanese Patent KOKAI No. 58(1983)-77661) and Japanese Patent KOKAI No. 58(1983)-77660 and the like. The fundamental construction of the above analytical elements is composed of a light-transmissive, liquid-impermeable support, an ammonia indicator layer containing an indicator which produces a detectable change by contacting ammonia, a liquid permeation barrier layer which is permeable to gaseous ammonia and substantially impermeable to liquid, a reaction layer containing a reagent which reacts with an ammonia-producing substrate to produce ammonia and a porous spreading layer laminated in this order. The integral multilayer analytical element disclosed in EP 0 204 334 A has a trapping function of the ammonia contained in a body fluid (endogeneous ammonia) in the analytical element itself, and the analytical element can remove the influence of the endogeneous ammonia. This analytical element is provided with an endogeneous ammonia-trapping layer which conducts an ammonia-trapping reaction above the layer generating ammonia through an ammonia-producing reaction in contact therewith. The integral multilayer analytical element disclosed in EP 0 287 112 A is provided with a layer having a diffusion-preventing ability which does not perform trapping of ammonia and ammonia-producing reaction between the endogeneous ammonia-trapping layer and the ammonia-producing reaction reagent layer.
Heretofore, a copolymer latex of polyvinyl acetate-acrylate ester was used as the binder of the indicator layer. However, the analytical element using the above copolymer latex is insufficient in sensitivity, and the accuracy (CV=variation coefficient) is inferior. Since the coating solution is latex, it is inferior in the liquid stability because of the occurrence of precipitation. Moreover, since a pH variation of the latex solution occurs, this analytical element is unsuitable for the system using a pH indiator.